1. Field of the Invention
The present invention relates to a constant pressure generator and in particular to one for use within a patient breathing system to deliver a constant pressure output of breathable gas for supply to the patient.
2. Description of the Prior Art
The term “patient breathing system” is used as a generic descriptor for a ventilator, respirator, anaesthetic, or other medical system used to supply a breathing gas to a patient, together with the associated respiration circuit required to guide and control the supply of the breathing gas to the patient.
Most hospitals usually receive breathing gas necessary for the operation of patient breathing systems from central, high pressure, sources. However most patient breathing systems are designed to operate with a breathing gas at pressures of between 30 cm to 100 cm H2O. Therefore any gas supplied from a central high pressure source requires a substantial pressure reduction before it can be passed to a patient. A pressure reduction also must usually be provided even when using bottled gas as the source.
Moreover, since the amount of breathing gas to be supplied to a patient through a patient breathing system needs to be accurately controlled, it is most desirable to provide a constant pressure gas supply within such a breathing systems.
Patient breathing systems are known in which a source of breathing gas is connected, usually via a valve, to a variable volume reservoir. The reservoir is subjected to a constant force that tends to reduce the volume of the reservoir as the amount of contained breathing gas reduces, to thereby maintain the gas at a known constant pressure over a more or less limited range of reservoir volumes. Gas may be removed from this constant pressure generator and supplied to the respiration circuit of the breathing system. Through careful choice of the constant force the generator can also provide an effective pressure reduction to the gas from the gas source.
A known constant pressure generator of this variable volume type is described in U.S. Pat. No. 3,824,902, which discloses a variable volume reservoir in the form of an expansion bellows. The bellows is made of a soft plastic material and positioned between two rigid plates. One plate is fixed in space while the other is mounted on a shaft for pivotal motion. The shaft is rotatable by means of an arm and spring arrangement to collapse the bellows. By suitable positioning of attachment points of the springs, together with a careful selection of the size of the springs, it is possible to maintain a constant force on the container over a limited operating range of reservoir volumes. This spring and arm arrangement, however, is relatively complex in construction and thus is expensive to manufacture.
Another constant pressure generator is disclosed in European Application 0 744 184 and has a variable volume reservoir in the form of a collapsible bellows. The walls of the bellows have a region consisting of an elastic material having an elasticity chosen to provide a constant force which causes the bellows to collapse and generate a constant pressure on breathing gas contained therein. The useful range of volume changes over which a constant pressure can be generated, however, is relatively small compared to the overall volume of the bellows. Additionally, position sensors on the bellows have to be used in association with control valves in order to ensure that gas is supplied from the reservoir only over the useful range. This also complicates the generator and together with the difficulties associated with fabricating a composite bellows also makes it expensive to construct.
A further constant pressure generator for use in a patient breathing system is described in European Application 0 557 134 and has a reservoir in the form of a collapsible bellows which is arranged inside a further chamber in a so-called “bag and bottle” arrangement. A propelling gas is connected to the further chamber via control valves which operate to control the flow of propelling gas into and out of the chamber in order to maintain a constant force on the outside of the bellows, at least during an inspiration phase of a patient's breathing cycle. The bellows tends to collapse under this force and so maintains the breathing gas contained therein at a constant pressure during the inspiration phase. In order to permit exhalation by the patient the propelling gas is removed from the further chamber, reducing the pressure therein and permitting the bellows to expand. Gas may then flow from the lungs of the patient. This bag and bottle arrangement is relatively complex in construction and operation.